Transceiver and System Design for Digital Communications, 5th Edition

Buy e-book PDF

This applied engineering reference covers a wide range of wireless communication design techniques; including link budgets, error detection and correction, adaptive and cognitive techniques, and system analysis of receivers and transmitters. Digital modulation and demodulation techniques using phase-shift keyed and frequency hopped spread spectrum systems are addressed. The book includes sections on broadband communications and home networking, satellite communications, global positioning systems (GPS), search, acquisition and track, and radar communications. Various techniques and designs are evaluated for modulating and sending digital signals, and the book offers an intuitive approach to probability plus jammer reduction methods using various adaptive processes. This title assists readers in gaining a firm understanding of the processes needed to effectively design wireless digital communication and cognitive systems with a basic understanding of radar. Transceiver and System Design for Digital Communications has been fully revised and updated in this new fifth edition, with the addition of two new chapters addressing radar communications and volume search and track. Derived from numerous training workshops taught to engineers through private courses by the authors, this book will appeal to digital wireless communications system designers in both the commercial and military sectors, in particular new engineers requiring practical design techniques and fundamental understanding of modern systems that employ digital transceivers.

A transceiver is a system that contains both a transmitter and a receiver. The transmitter from one transceiver sends a signal through space to the receiver of a second transceiver. After receiving the signal, the transmitter from the second transceiver sends a signal back to the receiver of the first transceiver, completing a two-way communications data link system, as shown in Figure 1.1. There are many factors to consider when designing a two-way communications link. The first one is to determine the operating frequency. Several consideration need to be evaluated to select the frequency that is going to be used.

The transmitter is responsible for formatting, encoding, modulating, and upconverting the data communicated over the link using the required power output according to the link budget analysis. The transmitter section is also responsible for spreading the signal using various spread spectrum techniques. Several digital modulation waveforms are discussed in this chapter. The primary types of digital modulation using direct sequence methods to phase modulate a carrier are detailed, including diagrams and possible design solutions. A block diagram showing the basic components of a typical transmitter is shown in Figure 2.1.

The receiver is responsible for downconverting, demodulating, decoding, and unformatting the data received over the link with the required sensitivity and bit error rate (BER) according to the link budget analysis of Chapter 1. The receiver is responsible for providing the dynamic range (DR) to cover the expected range and power variations and to prevent saturation from larger power inputs and provide the sensitivity for low-level signals. The receiver provides detection and synchronization of the incoming signals to retrieve the data sent by the transmitter. The receiver section is also responsible for despreading the signal when spread spectrum signals are used. The main purpose of the receiver is to take the smallest input signal, the minimum detectable signal (MDS), at the input of the receiver and amplify that signal to the smallest detection level at the analog-to-digital converter (ADC) while maintaining a maximum possible signal-to-noise ratio (SNR). A typical block diagram of a receiver is shown in Figure 3.1. Each of the blocks will be discussed in more detail.

Automatic gain control (AGC) is used in a receiver to vary the gain to increase the dynamic range (DR) of the system. AGC also helps deliver a constant amplitude signal to the detectors with different radio frequency (RF) signal amplitude inputs to the receiver. AGC can be implemented in the RF section, the intermediate frequency (IF) section, in both the RF and IF portions of the receiver, or in the digital signal processing (DSP) circuits. Digital AGCs can be used in conjunction with RF and IF AGCs. Most often, the gain control device is placed in the IF section of the receiver, but placement depends on the portion of the receiver that limits the DR. The detection of the signal level is usually carried out in the IF section before the analog-to-digital converter or analog detection circuits. Often the detection occurs in the DSP circuitry and is fed back to the analog gain control block. The phaselocked loop (PLL) is analyzed and compared with the AGC analysis, since both processes incorporate feedback techniques that can be evaluated using control system theory. The similarities and differences are discussed in the analysis. The PLL analysis is used only for tracking conditions and not for capturing the frequency or when the PLL is unlocked.

The demodulation process is part of the receiver process that takes the downconverted signal and retrieves or recovers the data information that was sent. This demodulation process requires three basic functions to retrieve the sent data: Recover the carrier, since the digital modulation results in a suppressed carrier and the carrier is recovered to remove it from the incoming signal, remove spread spectrum coding, if using spread spectrum techniques to mitigate jammers; generally done using a despreading matched filter correlator or a sliding correlator, align and synchronize the sample point for sampling the data stream at the optimal signal-to-noise ratio (SNR) point, which requires lining up the bits with the sample time using a bit synchronizer or over sampling the return. The process detects the digital data that was sent from the transmitter with a minimum bit error rate (BER). This was performed in the past using analog circuitry, such as mixers and filters to remove the carrier frequency and spread spectrum codes, but today, the process is incorporated in the digital circuitry using application-specific integrated circuits, field-programmable gate arrays, and digital signal processing (DSP)-integrated circuits.

To achieve a better understanding of digital communications, some basic principles of probability theory need to be discussed. This chapter provides an overview of theory necessary for the design of digital communications and spread spectrum systems. Probability is used to calculate the link budget in regard to the error and required signal-to-noise ratio (SNR) and to determine whether a transceiver is going to work and at what distances. This is specified for digital communications systems as the required Eb/No.

Multipath affects the desired signal by distorting both the phase and the amplitude. This can result in a lost signal or a distortion in the TOA of the desired signal. Multipath is divided into two categories: specular and diffuse. Specular multipath generally affects the system the most, resulting in more errors. Diffuse multipath is more noise-like and is generally much lower in power. The Rayleigh criterion is used to determine if the diffuse multipath needs to be included in the analysis. The curvature of the earth can affect the analysis for very long-distance multipath. One of the ways to reduce the effects of multipath is to use leading edge tracking so that most of the multipath is ignored. Some approaches for determining multipath effects include vector analysis and power summation. Several methods of multipath mitigation were discussed, including using multiple antennas for antenna diversity.

The receiver is open to reception of not only the desired signal but also all interfering signals within the receiver's bandwidth, which can prevent the receiver from processing the desired signal properly (Figure 8.1). Therefore, it is crucial for the receiver to have the ability to eliminate or reduce the effects of the interfering signals or jammers on the desired signal. These are divided into two groups, cosite and friendly jammers, which are often referred to as interferers, and unfriendly jammers that purposely jam the signal. The focus of this chapter is to address the types of unfriendly jammers and ways to mitigate their effect on the desired communication signal.

Cognitive systems are used to adapt the operating system to the changing environment. This can be accomplished by many methods suggested in this chapter, and many additional methods will be available in the future. The optimal cognitive system solution evaluates a system's available capabilities as well as all of the available knowledge about the changing environment, and then it calculates, makes trade-offs, and determines the best solution for the system to adapt to these environmental changes with minimal impact to performance. In addition, the cognitive system contains a learning capability that uses past experiences and impact/results of changes and uses this information to make smart decisions in the future. Game theory has been used to establish cognitive networks, and selfish nodes are eliminated by cooperation and establishing a Nash equilibrium. Although there are challenges with cognitive systems, they are beginning to infiltrate our current technologies, and they produce optimal communications and network systems.

Wireless communication systems that contain users that are moving in position and incorporate directional antennas require volume search, acquisition, and tracking methods to discover and maintain the communication link. Volume search is used to discover the user, acquisition is used to acquire the user by narrowing down the position of the user in order to point the antenna in the direction of the user, and tracking is used to lock on to the user's position and maintains the position as the user is moving in order to point the antenna in the direction of the user. There are several methods used to implement volume search, acquisition, and track depending on the requirements and capabilities of each system.

Broadband and home networking will shape the future. New standards are being reviewed as new technologies are developed and as the data rates increase. With several incoming signals to a home, such as voice, data, and video, there is a need to provide optimal distribution throughout to allow for easy access. Commercial and military communications are ubiquitous and constantly growing and improving using new technologies. Networking is becoming important on the military battlefield, and JTRS and Link 16 play important roles in the interoperability of communication devices. The development of these and other new technologies will provide the military with a network for all communication devices present and in the future.

Satellite communications are becoming a viable means of providing a wide range of applications for both the commercial and military sectors. The infrastructure for distributing signals covers the widest range of communications methods; even the most remote places on earth like the South Pole can have communications via satellite. Satellite's bandwidth, field of view, and availability, along with combining this technology with other types of communications systems, produce a ubiquitous infrastructure that provides communications worldwide.

The last few years have shown an increased interest in the commercialization of the global navigation satellite system (GNSS), which is often referred to as the global positioning system (GPS) in various applications. A GPS system uses spread spectrum signals-binary phase-shift keying (BPSK)-emitted from satellites in space for position and time determinations. Until recently, the use of GPS was essentially reserved for military use. Now there is great interest in using GPS for navigation of commercial aircraft. The US Federal Aviation Administration implemented a wide area augmentation system for air navigation to cover the whole United States with one system. There are also applications in the automotive industry, surveying, and personal and recreational uses.

Radio detection and ranging (RADAR) is a method of using electromagnetic waves to determine the position of a target. Radar transmits a signal and receives and detects a portion of the signal that is reflected back to the radar. Radar uses this return signal or echo to measure the time it is transmitted to the time it is received to determine the range of the target. In addition, the returned signal can be received by the radar antenna to determine the angle it received. Therefore, radar can determine the range and direction, velocity, and identifying characteristics of targets by monitoring the reflected signals coming back to the radar.

Direction finding is a method to determine the direction of a transmitted signal by using two antennas and measuring the phase difference between the antennas (Figure 15.1). This process is called interferometry. In addition to using a static interferometer, further analysis needs to be done to calculate the direction when the interferometer baseline is dynamic; that is, the interferometer is moving and rotating in a three-dimensional plane. Thus, coordinate conversion processes need to be applied to the nonstabilized antenna baseline to provide accurate measurement of the direction in a three-dimensional plane.